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drilling a porcelain sink

OK, this isn't exactly a SMC topic, but then again, what is?

I am installing a faucet for a water filter and need to drill a hole in the sink. It's porcelain coated, cast-iron. The directions for the faucet state about 5 times NOT to drill through a porcelain sink. I have, however, found tons of info about doing it and no real problems. Everyone says to use a diamond hole saw to get through the porcelain and then a bi-metal hole saw to get through the metal. I can't for the life of me figure out why the faucet instructions are so adamant about not drilling porcelain. Has anyone done it? Any "gotchas" to look out for?

Thanks!
Gary

I have done so much with so little for so long, that I can do almost anything with practically nothing...

They don't want some yahoo get in there with a split-point bit and start hacking away... porcelain is easily chipped, so they don't want someone bringing back a sink that has chipped porcelain and rusted cast iron underneath. Once you make a clean cut down to the metal you shouldn't have any issue, just go slow, and seal the cut edge as soon as possible to prevent rust.

Re: "faucet instructions are so adamant about not drilling porcelain"?..

If your goal is simply to "get'r done", get paid, and be gone that's fine. If you choose to stand behind quality workmanship for any extended period, you can likely anticipate that the hole you've made boring through the iron substrate may begin to rust. As iron oxide occupies a greater molecular volume than iron interspersed with carbon; moisture induced oxidation (rust) will progress radially through the iron, flaking off the porcelain coating, wrecking and discoloring the finish in proximty to that hole. You could avoid this problem by carefully recoating naked iron around the bore with some non-polar solvent based finish. Suggest nail polish, thinner-based polyurethane, 100% silicone caulking or a paintable water-proof elastomer or asphalt sealant. Knowing that few installers will make any effort to properly seal an incursion into the iron base when it is more readily just hidden from view, they opt to strongly advise against this product warranty violation. A reputable manufacturer may be approached to warranty a product failure long after the installer has left the scene.

Morey,
Thanks for the reply. I'm doing this for myself, but it wouldn't matter if it were for me or someone else, I would still want to do it properly. Your explanation makes perfect sense and I will plan on sealing the edges as you suggest, I wouldn't want to replace the sink a few years down the road because I took a shortcut now.

Thanks!
Gary

I have done so much with so little for so long, that I can do almost anything with practically nothing...

When you start cutting the cast iron, do not use any type of cutting fluid. Cast Iron should be cut dry to prevent the buildup of a sticky slurry that generally makes a mess of things. If you feel you need a coolant, put a vaccum cleaner nozzle near the cut. This will pick up the dust and cool the cutter. BTW, some cast iron is very hard because when chilled, it can produce carbides that will eat up any type of cutter.